Weak field polarization detection technique is one of detection techniques of the molecular alignment. This method does not damage the quantum state of the field-free aligned molecules, thus it is convenient for the further applications of these aligned molecules. The principles and the experimental schematics of the weak field polarization technique, and its improvement-balanced weak field polarization detection technique were introduced in detail. Based on the Taylor series approximation, the sensitivity of these two techniques were analyzed and compared. The original noises of these two techniques were investigated as well. The study found that the sensitivity of the latter is higher than the former when the alignment degree of the molecules is lower. The noise of the latter is lower than the former when the fluctuation of the laser pulse energy is obvious. The noises of the two techniques are close when the fluctuation of the laser pulse energy can be ignored.